I can confirm the southbound side appears to be about a foot wider than northbound. This is odd because I thought northbound was the newer of the two spans.

If you are talking about the direction of interstate traffic, then the northbound span is in fact the older of the two. You can tell in the girder styles; the older one having riveted girders. The overall span is also narrower as well; 38 to 40 feet (hard to tell if you aren't looking for it).

As most of my bike trips to and from Portland are southbound on 205 and northbound on the Interstate Bridge, I usually just stick to the northbound (eastern/older) span. Personally, I am a tad more comfortable on this one, as the rail is suspended over the side and therefore not taking up space with concrete posts like the other span.

Mind you, this is just my opinion, and may not be applicable for your riding style.

I know this is a bit late so far as this thread is concerned, but on Wednesday, I finally got around to measuring the widths of the paths, and this is what I found:
Southbound (Newer): 48" minimum at auto gate, 60" typical at posts
Northbound (Older): 40" minimum at gate, 44" typical to railing

Mind you, that 40" is on the ground, making it the widest your trailer or trike could be. Above that point, however, there are a few more inches of room.

It might just be the connectivity and not having to cross through that tunnel, or possibly the fewer pedestrians, but I would still say that I prefer the older, narrow path for riding. I still advocate to just travel with the traffic on the bridge. It makes it less hazardous to others that way. While not signed, this tends to be the de facto rule. I would even dare to say that the relative number of cyclists going the wrong way in this case would be similar to that of the signed Hawthorne bridge.